Effect of mid-diestrus oxytocin treatment on early pregnancy in mare

Progesterone supplementation in early pregnancy is commonplace in equine reproduction. Recently, there has been some controversy questioning the beneficial effects of routine progesterone administration and rising concerns as an occupational hazard for humans. Although premature luteal regression and resulting low plasma progesterone concentrations is rare in the mare, select cases benefit from luteal support. Luteal function can be extended following oxytocin administration during mid-diestrus. Aim of this field study was to investigate the effects of oxytocin administration on early pregnancy in the mare. Thoroughbred mares (n = 39) were enlisted and bred over 55 estrous cycles. Prior to natural breeding, uterine cultures were obtained and mares were assigned to 1 of 2 groups: Oxytocin treatment (OT); oxytocin 60 units, IM, once daily, days 7 - 14 postovulation; or Control (CT), given no treatment. Mare reproductive cycles were monitored by transrectal palpation and ultrasonography. Ovulation was documented and defined as day 0. Pregnancy diagnosis was performed by transrectal ultrasonography 14 days postovulation. If pregnancy was confirmed, a blood sample was collected and assayed for plasma progesterone concentrations. Mares that did not conceive were rebred and treatment was determined by the stage in the study; therefore, some mares underwent the same treatment more than once. Subsequent pregnancy examinations were performed on days 28 and 45 of pregnancy. First and percycle pregnancy rates for OT and CT were 90.9 and 91.3% and 68.2 and 55.1%, respectively. Median progesterone concentrations for OT were 6.9 and 7.0 ng/ml for CT. There were no differences between OT and CT groups for first (p = 0.165) or percycle (p = 0.289) pregnancy rates, days 28 and 45 pregnancy rates (p = 1.0) or progesterone concentrations (p = 1.0). Oxytocin administration during mid-diestrus in early pregnancy did not induce a negative effect. Further investigation in this area may reveal potential beneficial effects of oxytocin administration in early pregnancy.

Keywords: Mare, oxytocin, corpus luteum, progesterone, breeding management

This manuscript was originally published in the journal Clinical Theriogenology Vol 12(3) Sept 2020.  Clinical Theriogenology is the official journal of the Society for Theriogenology (SFT) and the American College of Theriogenologists (ACT).  This content has been reproduced on the IVIS website with the explicit permission of the SFT/ACT.